Traditional chrome plating creates a smooth, bright chrome finish. It has long been desired to have tinted lustrous metallic finishes, such as black chrome, gold-tinted chrome, and other tinted or colored metallic finishes for decorative purposes. More specifically, there has been a desire for clear polymeric decorative coatings that allow the brightness and luster of chrome plating to be seen, while modifying the appearance by imparting a color or tint to the chrome plating.
Several methods have been used in an attempt to achieve colored metallic finishes. One method involves electroplating, wherein black chrome and bronze finishes can be created. A problem with the electroplating methods is that it is very difficult to consistently obtain a desired colored metallic finish. Another disadvantage with the electroplating techniques is that they are generally limited to black and bronze colors or combinations thereof.
Another method that has been used for obtaining colored metallic finishes involves physical vapor deposition (PVD). This method offers a wider variety of colors than the electroplating method. However, physical vapor deposition is very expensive, and therefore, its use for achieving a desired colored finish is extremely limited. Another problem with physical vapor deposition is that it is difficult to control the gloss of the finishes.
Another method that has been attempted for obtaining colored metallic finishes involves applying transparent organic polymer coating containing a dye, pigment or other colorant on a chrome-plated substrate. This method is convenient, and offers a very wide variety of color finishes. However, the use of organic coatings on chrome-plated substrates has been extremely limited due to poor adhesion between the organic coating and the chrome surface, especially after prolonged exposure to extreme temperature and/or humidity. Even organic polymer coating compositions which initially exhibit good adhesion have not exhibited adequate adhesive durability for most product applications. As a result, this technique has not been applied to automotive components.
A process for adhering a relatively thick layer (e.g., 5 millimeters) of polyurethane elastomer to a metal, such as steel, iron or aluminum, is disclosed in U.S. Pat. No. 4,542,070. The process involves coating the surface of the metal with a primer composition containing a polyepoxy compound and a polyamine compound, further coating the surface with a composition containing an isocyanate compound having an isocyanate group concentration of 15-50% by weight, and casting a layer of polyurethane elastomer onto the double coated surface of the metal followed by hardening the whole system. A silane-coupling agent may be added to the primer composition to improve adhesive properties and water resistance. Disclosed examples of silane-coupling agents include gamma-glycidoxypropyltrimethoxysilane and gamma-aminopropyltriethoxysilane. Color pigments may be added to the composition. Solvents that may be used for the primer composition are those which dissolve both the polyamine compound and the polyepoxy compound, with examples including toluene, xylene, ethylbenzene, methylethylketone, methylcellosolve, ethylcellosolve and acetate esters of a cellosolve compound. The polyurethane elastomer is used to improve the durability of steel, iron and aluminum surfaces of metal articles.
U.S. patent application Ser. No. 09/707,866 describes a process for creating unique surface finishes on chrome-plate substrates. The process provides an economical way of creating surface finishes similar to black chrome, and other colored metallic finishes. The process employs an adhesion enhancer that can be applied as a primer or as an additive to a polyurethane composition. More particularly, in one embodiment, the process includes steps of applying an aqueous primer composition to a chrome substrate, wherein the primer composition containing a silane adhesion promoter; drying the applied primer composition; applying a urethane composition over the chrome plate on which the aqueous primer was applied and dried; and curing the urethane composition to form a polyurethane film. Both the film and the adhesion between the film and chrome plating are durable enough for practical applications such as in the automotive interior industry and in electronic devices such as cellular telephones. However, the processes described in this reference do not consistently provide the required adhesive durability needed for exterior automotive applications. Therefore, a process for forming a polymer film on a metal surface to provide a coating or tinted coating which exhibits consistently improved adhesive durability suitable for exterior automotive applications and other applications in which the coating is subjected to weather or other extreme conditions is desired.